High dielectric constant (HDC) materials have many microelectronic applications, such as DRAMs, embedded DRAMs, SRAMs, FeRAMS, on-chip capacitors and high frequency capacitors. Typically, these applications employ HDC materials in a capacitive structure, although the present invention may be used to make an HDC thin film with improved properties which is not part of a capacitor.
To facilitate construction of larger DRAMs with correspondingly smaller memory cells, capacitor structures and materials which can store the necessary charge in smaller spaces are needed. One of the most promising avenues of research to achieve this goal is the area of HDC materials. HDC materials generally have dielectric constants greater than 50. Examples of particular HDC materials are metal oxide materials such as, lead zirconate titanate (PZT), barium titanate, strontium titanate (SrTiO.sub.3), and barium strontium titanate (BST). It is desirable that such a material, if used for DRAMs and other microelectronics applications, be formable over an electrode and underlying structure (without significant harm to either), have low leakage current characteristics and long lifetime, and, for most applications, possess a high dielectric constant. The present invention relates to a method of forming a doped HDC film, specifically a barium and/or strontium titanate dielectric with improved resistance degradation and reduced film leakage.
While BST materials have been manufactured in bulk form previously, the physical and electrical properties of the material is not well understood when BST is formed as a thin film (generally less than 5 um) on a semiconducting device. while the dielectric constant of undoped bulk BST is maximized for median grain sizes of between about 0.7 um and about 1.0 um, for smaller grain sizes the dielectric constant fails off rapidly. Thus, BST having extremely small grain size is usually undesirable. Unfortunately, in submicron microcircuits such as DRAM capacitors, particular constraints are placed on BST grain size in the thin film. First, the annealing temperature for BST thin films must generally be kept far below the temperatures commonly used for sintering bulk BST ceramics (generally less than 700.degree. C. vs. typically greater than 1100.degree. C. for bulk BST) to avoid damage to the underlying device structure. Thus, the grain nucleation and growth kinetics of the BST crystal lattice is inhibited resulting in smaller grain sizes. Second, the desired film thickness in microelectronic applications may be much less than 5 um (preferably between about 0.05 um and about 0.1 um). It has been found that median grains sizes generally less than half the BST film thickness are required to control dielectric uniformity and avoid shorted capacitors. Thus, a method for producing a HDC material in a thin film structure having good dielectric properties is needed.
Generally, the introduction of dopant materials has been shown to affect the dielectric properties of the HDC thin film materials. Doped metal oxide materials, such as BST, in a MOCVD or sol-gel doping process are known to be useful in the manufacture of integrated circuit thin film capacitors having high dielectric constants. See for example, U.S. Pat. No. 5,122,923, herein incorporated by reference. However, while the effects of some dopants is known, dopant chemistry is far from an exact science.
It is known to be much harder to accurately control and predict the electrical properties of doped metal oxides as compared with traditional doped materials. Further, the usual methods of making oxides, such as BST, i.e. sputtering and pressing of powders, are inherently difficult to exactly control. In both formation processes, the dopants tend to be more highly concentrated in some parts of the film than in other areas of the film.
Prior methods used to dope the (Ba,Sr) TiO.sub.3 material utilized appropriate precursors of the possible dopants in a metal organic chemical vapor deposition (MOCVD) process. Typical MOCVD deposition of BST utilizes the precursors of Ba(bis(2,2,2,6-tetramethyl-3,5-heptanedionate)).sub.2 -tetraethylene glycol dimethyl ether; Sr(bis(2,2,2,6-tetramethyl-3,5-heptanedionate)).sub.2 -tetraethylene glycol dimethyl ether and Ti(bis(isopropoxy)).sub.2 bis(2,2,2,6-tetramethyl-3,5-heptanedionate).sub.2. A liquid delivery system mixed, metered and transported the precursors at room temperature and high pressure to a heated zone, where the precursors were then flash vaporized and mixed with a carrier gas, typically argon, to produce a controlled temperature, low pressure vapor stream. The gas stream was then flowed into a reactor mixing manifold where the gas stream mixed with oxidizer gases. Typically the oxidizer gases were O.sub.2 & N.sub.2 O. The mixture of the gas stream and the oxidizer gases then passed through a shower head injector into a deposition chamber. In the MOCVD deposition, both the ratio of the concentrations of the metalorganic compounds in the vaporized liquid and the deposition conditions determine the final film stoichiometry.
As can be realized, three component MOCVD process (for BST thin films) is quite complicated. Adding one or more additional components to the process (such as Nb, Ta or Sb dopants) will generate more complexity in the process chemistry. Additionally, developing the precursor chemistry of the multicomponent system to dope BST is also an important requirement.
A second prior method for doping BST is using a sol-gel process. Studies on a BST sol-gel process have been undertaken. These studies seem to show that doping of BST or other perovskite titanates via sol-gel process is possible. However, for DRAM technology, sol-gel technology for doping BST films is not a viable deposition process since step coverage with 10 to 1 aspect ratios can not be achieved with the sol-gel processes. Thus, when doping BST in a sol-gel process it is difficult to accurately dope the BST film along the sidewalls of a deep trench capacitor or a tall stud capacitor.
Therefore, in the MOCVD and sol-gel processes, doping BST with higher valence cations, such as, for example, Nb, Ta La or Sb, is challenging. Having stable precursors of such elements, adequate incorporation efficiency, uniformity in deposited films and control of stoichiometry in BST with process conditions still need to be explored. The present invention overcomes the difficulties of MOCVD and sol-gel doping of BST thin film dielectrics by using an ion implantation method to dope the BST thin film.